1. Field of the Invention
The present invention relates in general to the field of accelerometers and in particular to the field of vertically sensitive accelerometers. Still more particularly the present invention relates to self-orienting vertically sensitive accelerometers useful for measuring a vertical component of acceleration independently of the orientation of the accelerometer.
2. Description of the Prior Art
Offshore seismic exploration typically employs elongate marine streamer cables in tow behind an exploration vessel. Such streamers typically contain hydrophones, or transducers which are responsive to variations in pressure. Each streamer is generally fluid-filled to provide a medium for sensor coupling. The hydrophones are utilized as receivers for reflected seismic waves which are generated by a seismic source, such as air gun array. The ability to simultaneously record vertical particle motion and pressure has long been recognized as a method for improving signal to noise ratio in seismic recording. For example, see U.S. Pat. No. 2,757,356, issued July 31, 1956 entitled Method and Apparatus for Cancelling Reverberations in Water Layers or U.S. Pat. No. 4,486,865, issued Dec. 4, 1984, entitled Pressure and Velocity Detectors For Seismic Exploration.
One problem which exists in marine seismic exploration involves so-called ghosts or spectral notches which are generated as up-going seismic energy is reflected from the water/air interface at the surface of a body of water. The reflection coefficient at the surface is nearly unity in magnitude and negative in sign. Therefore, when hydrophones are positioned at integer wave lengths in depth, a notch in the power spectrum will occur at the corresponding frequency as the upward energy is cancelled by the reflected energy from the water/air interface.
For particle velocity however the reflection coefficient is positive at the water/air boundary. Notches will therefore appear in a particle velocity amplitude spectrum when the hydrophones are positioned at half wave lengths in depth. By utilizing the fluid particle velocity in addition to the dynamic fluid pressure, separation of the up-going and down-going wave fronts may be achieved. Addition of the pressure and velocity measurements, scaled by the acoustic impedance of the fluid in which the transducers are immersed will generally yield spectra free of ghosts and deep spectral notches.
It is generally known that an accelerometer may be utilized in such applications, rather than a velocity transducer. The acceleration signal is then either electrically or numerically integrated to convert this signal to a signal which is representative of particle velocity. It is important that the accelerometers employed in such applications be insensitive to dynamic pressure changes in order to achieve maximum separation of the up-going wave front from the down-going wave front.
Proper orientation of an accelerometer or geophone axis in a marine streamer is non-trivial. Exploration streamers towed behind marine vessels are typically over one mile in length. Modern marine streamers may utilize in excess of 10,000 transducers. In order to maintain each accelerometer in a proper orientation to detect the vertical component of acceleration, the prior art has proposed various solutions. For example, the use of gimbals has been proposed repeatedly. Gimbaling generally requires the utilization of expensive bearings and commutators however and has not generally been successful. U.S. Pat. No. 4,618,949, issued Oct. 21, 1986, entitled Self-Orienting Directionaly Sensitive geophone, proposes a solution which utilizes a ferromagnetic fluid as a suspension means for a magnet, thereby permitting rotation of the device while maintaining vertical sensitivity. However, the aforementioned patent does not address the problem which occurs in a marine streamer which is not horizontally disposed.
U.S. Pat. No. 4,345,473, issued Aug. 24, 1982, entitled Vertical Component Accelerometer, offers a solution which includes concentric spheres or cylinders which are lined with piezoelectric ceramic material. The gap between the inner and outer shells is then partially filled with a conductive fluid. The interconnection and polarization of the inner and outer piezoelectric layers is then utilized to cause the pressure sensitivity of this device cancel. However, the fact that this device utilizes piezoelectric material in a compressional mode means that the device is fairly insensitive when compared to a device which utilizes piezoelectric material in a bending mode. To achieve a sufficiently high sensitivity to be of utilization in detecting seismic signals the apparatus proposed in U.S. Pat. No. 4,345,473 would require a device of such a large diameter so as to be impractical in modern small-diameter marine streamers. The utilization of a piezoelectric material in a compressional mode, as disclosed in U.S. Pat. No. 4,345,473, would mean that for a sphere of moderate size (approximately one inch in diameter) the noise floor of a typical charge amplifier would be much greater than the signal levels this device could generate when excited by a seismic wave from a moderate depth reflector. For this reason, piezoelectric hydrophones in current use operate with thin ceramic materials in a bending mode to achieve a sufficiently high sensitivity.
U.S. Pat. No. 4,789,971, issued Dec. 6, 1988, entitled Broad Band, Acoustically Transparent, Non-Resonant PVDF Hydrophone, teaches the utilization of a piezoelectric polymer film of polyvinylidene fluoride (PVDF) which may be operated in a tension mode to create a device having sufficiently high sensitivity.
Those skilled in the art will appreciate that in land seismic surveys thousands of geophones are typically deployed for each survey. Each geophone is manually planted into the ground and rapid deployment is necessary for low cost operation. Therefore, care is not always taken to ensure that all seismometers are vertically oriented. Although most geophones in current use are of the moving coil type wherein a tilt of less than 15.degree. is of minor consequence, larger angles may cause decreased sensitivity due to coil drag. Therefore, a self-orienting vertical seismometer would be of value for land applications as well.